Capturing transient scenes at a high imaging speed has been pursued by photographers for centuries, tracing back to Muybridge's 1878 recording of a horse in motion and Mach's 1887 photography of a supersonic bullet. However, not until the late 20th century were breakthroughs achieved in demonstrating ultra-high speed imaging (>100 thousand, or 105, frames per second). In particular, the introduction of electronic imaging sensors, such as the charge-coupled device (CCD) and complementary metal-oxide-semiconductor (CMOS), revolutionized high-speed photography, enabling acquisition rates up to ten million (107) frames per second. Despite the widespread impact of these sensors, further increasing frame rates of imaging systems using CCD or CMOS are fundamentally limited by their on-chip storage and electronic readout speed.
The formation of a photonic Mach cone by a superluminal light source in a medium—i.e., a source traveling faster than the speed of light in that medium—can be theoretically predicted. Yet, thus far, photonic Mach cones have eluded experimental visualization owing to the challenges in producing superluminal light sources and achieving light-speed imaging at sufficiently high framing rates.
When an object moves supersonically in air, the induced pressure wave eventually develops into a shock wave because higher-amplitude pressure travels faster. Manifesting as an abrupt wavefront, the shock wave is heard as a sudden “crack” or “boom”, called a sonic boom 1. The wavefront forms a salient cone—defined as the Mach cone—with the vertex anchored at the object. Mach cones have been observed with various supersonic objects, such as aircraft and bullets. A Mach cone can be created with a moving source in any other medium provided that the source's speed exceeds the propagation speed of the excited waves in the same medium. For instance, Mach cones have been observed in the cases of a moving ship in water, a moving electron in plasma, and a moving magnetic field pulse in ferromagnets.
Although superluminal (i.e., faster-than-light) travel in vacuum is forbidden by Einstein's special theory of relativity, a superluminal light source may be generated in a medium with a refractive index greater than unity. For example, a high-energy charged particle (such as an electron) barreling through a medium can polarize the medium along its track and thereby trigger a cascade of photon emissions. When the speed of this high-energy charged particle exceeds the speed of light in this medium, these emitted photons constructively interfere with each other, forming so-called Cherenkov radiation—the photonic equivalent of a sonic boom. The Cherenkov-radiation-induced photonic Mach cone is important in disciplines such as particle physics, astrophysics, and medical imaging. Because light travels orders of magnitude faster than sound, a photonic Mach cone is much more challenging to produce in a standard laboratory setting and to observe in real-time (defined as the actual time during which a single event occurs) than the sonic counterpart. To confirm experimental generation of the photonic Mach cone using a superluminal light source, an imaging method that is sufficiently fast to enable real-time visualization of the cone is needed.